Blender Multiple Bake Problem
I was able to bake three objects on one UV Map texture by doing the following:


In UV Image Editor, create a new image texture. I did a simple black 1024x1024.
Select all three objects. Tab into Edit Mode. Unwrap "Light Map Pack" for quick and dirty testing. You will see the unwrap of all three objects evenly distributed across the UV image.
Create materials for your objects.
Add a "Image Texture" node to each object material (but do not connect it to anything). Make sure the Image Texture node has the UV map you created for baking in it. You should see "3" users for that Image Texture node. Make sure that node is selected / highlighted in each object.
Select your objects and hit bake.